A method and system are disclosed for exciting at least one electrode of a capacitively coupled reactive plasma reactor containing a substrate.
The present invention may advantageously be applied to:                Plasma-enhanced Chemical Vapour Deposition (PECVD) of thin films, for example of amorphous, micro- and nano-crystalline alloy of silicon and/or germanium and/or carbon for photovoltaic solar cell manufacture; also deposition of other thin films such as SiO2, Si3N4 etc for flat-panel display and integrated circuit manufacture;        Plasma etching of thin films, including Si, SiO2, Si3N4, metals etc for FPD manufacture, integrated circuit manufacture, photonic device manufacture; and        Other plasma surface modification processes such as ion implantation, surface modification, hardening etc . . .        
In general, parallel-plate reactors excited by capacitively-coupled radio-frequency (RF) power, with frequency from 0.1 to 200 MHz for example, are widely employed for depositing, etching and modifying thin films in domains including microelectronics and solar panel manufacture.
In large-area plasma processing (deposition or etching) of substrates in parallel-plate capacitive plasma reactors using conventional sinusoidal excitation, the plasma density and therefore the deposition or etch rate, can only be increased by increasing the applied radiofrequency voltage. This simultaneously increases the average energy of ions striking the substrate. In many applications, such as PECVD of silicon thin films, excessive ion energy damages the substrate or deposited film, so that only low voltages can be used, resulting in limited processing rates. Furthermore, the shape of the ion energy distribution function (IEDF) in conventional sinusoidally excited reactors is complex, typically saddle-shaped, and cannot be controlled.
Historically, sinusoidal waveforms have been used, first single frequency, and more recently two or three-frequency, but without synchronization of the different generators. Optimal processing of the substrate requires complete control of the energy of ions arriving at its surface, independent of the ion flux. For example, for etching high ion energies are needed, whereas for silicon deposition the ion energy must be kept below a threshold value around tens of electron volts.
Some independent control of ion flux and energy has been achieved by varying the RF frequency: at higher frequency the sheath impedance is lower, giving higher plasma densities, higher ion fluxes and lower ion energies for a given RF input power. However, in a symmetrical reactor, which has parallel electrodes of equal area, the equivalence of the sheaths in front of the two electrodes remains, so that both electrodes have equal ion bombardment. Further problems occur in large area high-frequency reactors due to standing wave effects, causing non-uniform processing across the wafer.
Document WO 2009/115135 proposes that by using two frequencies f+2f, of equal voltage amplitude the symmetry of the sheaths is broken, and the division of the voltage between the two sheaths should be continuously controllable by varying the phase delay between the two frequencies, allowing control of the ion energy at each surface.
In Patterson, M. M., H. Y. Chu, and A. E. Wendt, “Arbitrary substrate voltage wave forms for manipulating energy distribution of bombarding ions during plasma processing.”; Plasma Sources Science & Technology, 2007. 16(2): p. 257-264, the authors disclose that the control of ion energy distribution at a substrate in a decoupled plasma source such as an inductively coupled plasma (ICP) or helicon can be achieved by tailoring the wave form shape of a radio-frequency bias applied to the substrate, using a programmable waveform generator in combination with a power amplifier.
The present invention proposes to improve prior art techniques with simultaneous plasma generation in a parallel plate reactor and control of the Ion Energy Distribution Functions (IEDF), either mono-energetic or more complex, for better optimization of plasma etch and deposition processes.
An object of the present invention is an application of an electrical asymmetry effect in a capacitively coupled reactive plasma reactor.
Another object of the present invention is a complete control of the growth mode of thin films deposited at low temperature.